This invention relates to laminated electrically heatable windows, and more particularly to an improved bus bar termination for making electrical contact between an electrically conductive layer in the window and a supply of electric current.
Recently, electrically heatable windows have been used for defrosting or defogging windshields and back windows of automobiles. One type of electrically heatable window includes a very thin conductive metal film on a carrier film of transparent plastic embedded in the window and extending over most of the area of the window. The metal film is so thin it is transparent to light, and when an electrical current is passing between its opposite edges, the metal acts as a sheet resistor. The metal film carries enough current to heat the windshield to inhibit condensation of fog or frost and remove accumulations of ice or snow.
Typically, the thin film of conductive metal is vacuum-deposited on a thin carrier film of plastic such as polyethylene terephthalate, commercially available as Du Pont's "Mylar". The carrier film is sandwiched between a pair of transparent sheets of interlayer material typically polyvinyl butyral which are, in turn, sandwiched between exterior face sheets of glass. Electrically conductive bus bars, typically made of thin copper foil, are laminated into the heatable window between the vacuum coated side of the carrier film and the adjacent interlayer so they are in electrical contact with the thin conductive film.
Electric current applied between the conductive bus bars along opposite edges of conductive areas within the window assures uniform heating over the area of the window. The ends of the conductive bus bars are typically extended beyond the edge of the window for making electrical connection to a harness cable leading to the automobile alternator for supplying electric current to the window. In a typical prior arrangement, the ends of the foil bus bars extending beyond the edge of the window are substantially completely embedded in a flat, projecting laminated tab comprising relatively thin face sheets of plastic. The tab has small exposed areas above each bus bar for providing electrical contact between the bus bars and the harness cable.
Such electrically heatable windows for automobiles are normally manufactured by initially forming a "prelaminate" which includes the conductive carrier film, the plastic interlayers heat-bonded to opposite sides of the carrier film, and the bus bars embedded between the interlayers and the conductive film. The prelaminates then undergo a laminating process in which face sheets of glass are applied to the opposite sides of the prelaminate. The glass typically is applied by evacuating excess air out of the laminate and applying heat and pressure, such as by infrared heating or in an autoclave or by use of nip rolls, to tackify the interlayers to aid in bonding them to the glass. Thereafter, the assembly is put into an autoclave where heat and pressure are applied by an oil medium to complete bonding the interlayers to the glass sheets. The end-product from the laminating process comprises the laminated electrically heatable window having terminal connections for the embedded bus bars extending from a corner of the window in the projecting plactic tab described above.
The window is then shipped to the automobile assembly plant where the window is installed in the automobile. The terminal contacts of the bus bars are then connected to a harness cable for supplying electric current to the window heating element from the automobile alternator or generator. Typically, the contact on the end of the harness cable is a relatively complicated fitting which includes spring metal electrical contacts encased in an electrically insulated housing having a hinged closure. The bus bar tab at the corner of the window is fitted into the housing so the exposed areas of the contacts overlie the bus bars. The closure is then snapped to a closed position over the tab to apply pressure for holding the bus bars against the electrical contacts in the fitting. The harness terminals may be applied at the windshield laminating plant and shipped pre-assembled to the assembly plant.
It is important that the connection between the harness cable and the bus bar leads be in a waterproof seal, because this terminal connection, when installed in the automobile, is normally on the weather-side of the automobile firewall. The harness cable fitting described above relies on a sealing gasket and may be only partially waterproof.
It is also desirable for the bus bar termination to satisfy such requirements as (1) low cost, (2) good electrical contact after being subjected to the laminating process, (3) simplicity of attachment to the electrical cable harness, (4) minimal autoclave oil penetration which would disrupt the electrical contact, (5) adequate "pull" strength to resist tearing or separation of the bus bars, and (6) permitting the connector joint to be extended a substantial distance from the windshield, if desired. The harness cable fitting described above has adequate pull strength, but only partially satisfies these other requirements.
The present invention provides an improved bus bar termination which is completely waterproof and also meets all the requirements enumerated above.